Tandem catalyst converts propane to propylene

By combining the catalytic powers of platinum and indium oxide in finely tuned nanoparticles, researchers have improved a reaction that converts propane into propylene, an important process in the petrochemical industry (Science 2021, DOI: 10.1126/science.abd4441).

The proof-of-concept work shows that nanostructured tandem catalysts—which host different reactions side-by-side on a single nanoparticle—have the potential to play a bigger role in industrial processes, says Justin M. Notestein of Northwestern University, who led the work with his colleague Peter C. Stair.

The team targeted propylene production because of its significance in the chemical industry—global output reached 110 million metric tons last year, much of it destined for polypropylene plastics. Changes to the raw materials supplying steam crackers have shrunk the supply of propylene, while the method developed to replace this supply, propane dehydrogenation (PDH), is energy intensive and expensive. PDH plants convert propane to propylene at temperatures of 600 °C or more, and these conditions generate sooty carbon deposits that quickly deactivate the catalysts.

To solve these problems, researchers have spent decades developing the oxidative dehydrogenation of propane (ODHP), in which the hydrogen freed from propane is immediately combined with oxygen to create water. This pulls the reaction equilibrium in the right direction, requires lower temperatures, and makes less catalyst-corrupting carbon. “Given the scale at which this technology is needed, that could mean tremendous energy and cost savings,” says Ive Hermans of the University of Wisconsin-Madison, who has developed boron-based catalysts for ODHP. However, ODHP catalysts also have a nasty habit of turning propylene into CO and CO₂, which means they still cannot beat the propylene output from PDH.

That’s where the new tandem catalyst comes in. It contains two catalysts, each targeting a different stage of the reaction, to increase propylene production and reduce the formation of unwanted byproducts.

To make the catalyst, the Northwestern team dotted 2 nm wide clumps of platinum onto 100 nm particles of alumina. Then they coated each particle with a 2 nm thick shell of indium oxide, using a process called atomic layer deposition. Heating the particles opened up 1.4 nm pores in the shell, exposing about half of the platinum atoms on the surface beneath.

During the 450 °C ODHP reaction, platinum plucks hydrogen from propane to release propylene before indium oxide takes over to combine the hydrogen atoms with oxygen. This process converts about 40% of the available propane, giving a mix of products that is about 75% propylene and 25% CO₂, with virtually no carbon. Overall, Notestein says, this system offers the best balance between conversion and selectivity for any ODHP catalyst. “To be honest, I did not expect this system to work nearly as well as it did,” he says.

The indium oxide shell stabilizes the platinum nanoparticles, which improves the catalyst’s longevity. Since the reaction operates at a constant temperature in a single vessel, Notestein says it could enable much simpler reactor designs than typical PDH systems. “From what I’ve seen, it looks very exciting,” Hermans says. Notestein adds that tandem catalysts might also offer a less energy-intensive route to produce ethylene.

The tiny laboratory system used in this study is a long way from a gigantic propylene plant, though, and atomic layer deposition is a laborious way to build a catalyst. “A major challenge with the scale-up process will be the production of the catalyst,” says Jinlong Gong of Tianjin University, who has developed PDH catalysts. Notestein hopes that more straightforward synthesis methods could be developed to create similar nanostructures.